6-(1-Hydroxyalkyl)penam sulfone derivatives as inhibitors of class A and class C beta-lactamases I

Exploring Additional Dimensions of Complexity in Inhibitor Design for Serine β-Lactamases: Mechanistic and Intra- and Inter-molecular Chemistry Approaches

As a bacterial resistance strategy, serine β-lactamases have evolved from cell wall synthesizing enzymes known as penicillin-binding proteins (PBP), by not only covalently binding β-lactam antibiotics but, also acquiring mechanisms of deacylating these antibiotics. This critical deacylation step leads to release of hydrolyzed and inactivated β-lactams, thereby providing resistance for the bacteria against these antibiotics targeting the cell wall. To combat β-lactamase-mediated antibiotic resistance, numerous β-lactamase inhibitors were developed that utilize various strategies to inactivate the β-lactamase. Most of these compounds are “mechanism-based” inhibitors that in some manner mimic the β-lactam substrate, having a carbonyl moiety and a negatively charged carboxyl or sulfate group. These compounds form a covalent adduct with the catalytic serine via an initial acylation step. To increase the life-time of the inhibitory covalent adduct intermediates, a remarkable array of different strategies was employed to improve inhibition potency. Such approaches include post-acylation intra- and intermolecular chemical rearrangements as well as affecting the deacylation water. These approaches transform the inhibitor design process from a 3-dimensional problem (i.e., XYZ coordinates) to one with additional dimensions of complexity as the reaction coordinate and time spent at each chemical state need to be taken into consideration. This review highlights the mechanistic intricacies of the design efforts of the β-lactamase inhibitors which so far have resulted in the development of “two generations” and 5 clinically available inhibitors.

Detecting a quasi-stable imine species on the reaction pathway of SHV-1 β-lactamase and 6β-(hydroxymethyl)penicillanic acid sulfone

For the class A β-lactamase SHV-1, the kinetic and mechanistic properties of the clinically used inhibitor sulbactam are compared with the sulbactam analog substituted in its 6β position by a CH2OH group (6β-(hydroxymethyl)penicillanic acid). The 6β substitution improves both in vitro and microbiological inhibitory properties of sulbactam. Base hydrolysis of both compounds was studied by Raman and NMR spectroscopies and showed that lactam ring opening is followed by fragmentation of the dioxothiazolidine ring leading to formation of the iminium ion within 3 min. The iminium ion slowly loses a proton and converts to cis-enamine (which is a β-aminoacrylate) in 1 h for sulbactam and in 4 h for 6β-(hydroxymethyl) sulbactam. Rapid mix-rapid freeze Raman spectroscopy was used to follow the reactions between the two sulfones and SHV-1. Within 23 ms, a 10-fold excess of sulbactam was entirely hydrolyzed to give a cis-enamine product. In contrast, the 6β-(hydroxymethyl) sulbactam formed longer-lived acyl-enzyme intermediates that are a mixture of imine and enamines. Single crystal Raman studies, soaking in and washing out unreacted substrates, revealed stable populations of imine and trans-enamine acyl enzymes. The corresponding X-ray crystallographic data are consonant with the Raman data and also reveal the role played by the 6β-hydroxymethyl group in retarding hydrolysis of the acyl enzymes. The 6β-hydroxymethyl group sterically hinders approach of the water molecule as well as restraining the side chain of E166 that facilitates hydrolysis.

Inactivation of a class A and a class C β-lactamase by 6β-(hydroxymethyl)penicillanic acid sulfone

β-Lactamase inhibitors (clavulanic acid, sulbactam, and tazobactam) contribute significantly to the longevity of the β-lactam antibiotics used to treat serious infections. In the quest to design more potent compounds and to understand the mechanism of action of known inhibitors, 6β-(hydroxymethyl)penicillanic acid sulfone (6β-HM-sulfone) was tested against isolates expressing the class A TEM-1 β-lactamase and a clinically important variant of the AmpC cephalosporinase of Pseudomonas aeruginosa, PDC-3. The addition of the 6β-HM-sulfone inhibitor to ampicillin was highly effective. 6β-HM-sulfone inhibited TEM-1 with an IC(50) of 12 ± 2 nM and PDC-3 with an IC(50) of 180 ± 36 nM, and displayed lower partition ratios than commercial inhibitors, with partition ratios (k(cat)/k(inact)) equal to 174 for TEM-1 and 4 for PDC-3. Measured for 20 h, 6β-HM-sulfone demonstrated rapid, first-order inactivation kinetics with the extent of inactivation being related to the concentration of inhibitor for both TEM-1 and PDC-3. Using mass spectrometry to gain insight into the intermediates of inactivation of this inhibitor, 6β-HM-sulfone was found to form a major adduct of +247 ± 5 Da with TEM-1 and +245 ± 5 Da with PDC-3, suggesting that the covalently bound, hydrolytically stabilized acyl-enzyme has lost a molecule of water (HOH). Minor adducts of +88 ± 5 Da with TEM-1 and +85 ± 5 Da with PDC-3 revealed that fragmentation of the covalent adduct can result but appeared to occur slowly with both enzymes. 6β-HM-sulfone is an effective and versatile β-lactamase inhibitor of representative class A and C enzymes.

Modifications of the C6-substituent of penicillin sulfones with the goal of improving inhibitor recognition and efficacy

In order to evaluate the importance of a hydrogen-bond donating substituent in the design of β-lactamase inhibitors, a series of C6-substituted penicillin sulfones, lacking a C2' substituent, and having an sp(3) hybridized C6, was prepared and evaluated against a representative classes A and C β-lactamases. It was found that a C6 hydrogen-bond donor is necessary for good inhibitory activity, but that this feature alone is not sufficient in this series of C6β-substituted penicillin sulfones. Other factors which may impact the potency of the inhibitor include the steric bulk of the C6 substituent (e.g., methicillin sulfone) which may hinder recognition in the class A β-lactamases, and also high similarity to the natural substrates (e.g., penicillin G sulfone) which may render the prospective inhibitor a good substrate of both classes of enzyme. The best inhibitors had non-directional hydrogen-bonding substituents, such as hydroxymethyl, which may allow sufficient conformational flexibility of the acyl-enzyme for abstraction of the C6 proton by E166 (class A), thus promoting isomerization to the β-aminoacrylate as a stabilized acyl-enzyme.

Current challenges in antimicrobial chemotherapy: focus on ß-lactamase inhibition

The use of the three classical beta-lactamase inhibitors (clavulanic acid, tazobactam and sulbactam) in combination with beta-lactam antibacterials is currently the most successful strategy to combat beta-lactamase-mediated resistance. However, these inhibitors are efficient in inactivating only class A beta-lactamases and the efficiency of the inhibitor/antibacterial combination can be compromised by several mechanisms, such as the production of naturally resistant class B or class D enzymes, the hyperproduction of AmpC or even the production of evolved inhibitor-resistant class A enzymes. Thus, there is an urgent need for the development of novel inhibitors. For serine active enzymes (classes A, C and D), derivatives of the beta-lactam ring such as 6-beta-halogenopenicillanates, beta-lactam sulfones, penems and oxapenems, monobactams or trinems seem to be potential starting points to design efficient molecules (such as AM-112 and LK-157). Moreover, a promising non-beta-lactam molecule, NXL-104, is now under clinical development. In contrast, an ideal inhibitor of metallo-beta-lactamases (class B) remains to be found, despite the huge number of potential molecules already described (biphenyl tetrazoles, cysteinyl peptides, mercaptocarboxylates, succinic acid derivatives, etc.). The search for such an inhibitor is complicated by the absence of a covalent intermediate in their catalytic mechanisms and the fact that beta-lactam derivatives often behave as substrates rather than as inhibitors. Currently, the most promising broad-spectrum inhibitors of class B enzymes are molecules presenting chelating groups (thiols, carboxylates, etc.) combined with an aromatic group. This review describes all the types of molecules already tested as potential beta-lactamase inhibitors and thus constitutes an update of the current status in beta-lactamase inhibitor discovery.

Three decades of beta-lactamase inhibitors

Since the introduction of penicillin, beta-lactam antibiotics have been the antimicrobial agents of choice. Unfortunately, the efficacy of these life-saving antibiotics is significantly threatened by bacterial beta-lactamases. beta-Lactamases are now responsible for resistance to penicillins, extended-spectrum cephalosporins, monobactams, and carbapenems. In order to overcome beta-lactamase-mediated resistance, beta-lactamase inhibitors (clavulanate, sulbactam, and tazobactam) were introduced into clinical practice. These inhibitors greatly enhance the efficacy of their partner beta-lactams (amoxicillin, ampicillin, piperacillin, and ticarcillin) in the treatment of serious Enterobacteriaceae and penicillin-resistant staphylococcal infections. However, selective pressure from excess antibiotic use accelerated the emergence of resistance to beta-lactam-beta-lactamase inhibitor combinations. Furthermore, the prevalence of clinically relevant beta-lactamases from other classes that are resistant to inhibition is rapidly increasing. There is an urgent need for effective inhibitors that can restore the activity of beta-lactams. Here, we review the catalytic mechanisms of each beta-lactamase class. We then discuss approaches for circumventing beta-lactamase-mediated resistance, including properties and characteristics of mechanism-based inactivators. We next highlight the mechanisms of action and salient clinical and microbiological features of beta-lactamase inhibitors. We also emphasize their therapeutic applications. We close by focusing on novel compounds and the chemical features of these agents that may contribute to a "second generation" of inhibitors. The goal for the next 3 decades will be to design inhibitors that will be effective for more than a single class of beta-lactamases.

Regiospecific syntheses of 6alpha-(1R-Hydroxyoctyl)penicillanic acid and 6beta-(1R-hydroxyoctyl)penicillanic acid as mechanistic probes of class D beta-lactamases

The unique hydrophobic surface patches in class D beta-lactamases presented an opportunity for designing two compounds, 6alpha-(1R-hydroxyoctyl)penicillanic acid and 6beta-(1R-hydroxyoctyl)penicillanic acid, as mechanistic probes of these enzymes. In a sequence of three synthetic steps from benzhydryl 6,6-dibromopenicillanate, the targeted compounds were prepared in a stereospecific manner.

Approaches to the simultaneous inactivation of metallo- and serine-beta-lactamases

A series of cephalosporin-derived reverse hydroxamates and oximes were prepared and evaluated as inhibitors of representative metallo- and serine-beta-lactamases. The reverse hydroxamates showed submicromolar inhibition of the GIM-1 metallo-beta-lactamase. With respect to interactions with the classes A, C, and D serine beta-lactamases, as judged by their correspondingly low K(m) values, the reverse hydroxamates were recognized in a manner similar to the non-hydroxylated N-H amide side chains of the natural substrates of these enzymes. This indicates that, with respect to recognition in the active site of the serine beta-lactamases, the OC-NR-OH functionality can function as a structural isostere of the OC-NR-H group, with the N-O-H group presumably replacing the amide N-H group as a hydrogen bond donor to the appropriate backbone carbonyl oxygen of the protein. The reverse hydroxamates, however, displayed k(cat) values up to three orders of magnitude lower than the natural substrates, thus indicating substantial slowing of the hydrolytic action of these serine beta-lactamases. Although the degree of inactivation is not yet enough to be clinically useful, these initial results are promising. The substitution of the amide N-H bond by N-OH may represent a useful strategy for the inhibition of other serine hydrolases.

Understanding the longevity of the beta-lactam antibiotics and of antibiotic/beta-lactamase inhibitor combinations

Microbial resistance necessitates the search for new targets and new antibiotics. However, it is likely that resistance problems will eventually threaten these new products and it may, therefore, be instructive to review the successful employment of beta-lactam antibiotic/beta-lactamase inhibitor combinations to combat penicillin resistance. These combination drugs have proven successful for more than two decades, with inhibitor resistance still being relatively rare. The beta-lactamase inhibitors are mechanism-based irreversible inactivators. The ability of the inhibitors to avoid resistance may be due to the structural similarities between the substrate and inhibitor.

Penicillin-derived inhibitors that simultaneously target both metallo- and serine-β-lactamases

The synthesis and beta-lactamase inhibitory activity of four 6-(mercaptomethyl)penicillinates and the four corresponding 6-(hydroxymethyl)penicillinates are described. These penicillins include both C6 stereoisomers as well as the sulfide and sulfone oxidation states of the penam thiazolidine sulfur. All compounds were evaluated as inhibitors of representative metallo- and serine-beta-lactamases enzymes. Selected (mercaptomethyl)penicillinates are shown to inactivate both metallo- and serine-beta-lactamases and to display synergism with piperacillin against beta-lactamase producing strains.

Synthesis and biological evaluation of 6-bromo-6-substituted penicillanic acid derivatives as beta-lactamase inhibitors

The synthesis of a selected set of 6-bromopenicillanic acid derivatives with an additional C6 substituent is reported. All these substances were tested as inhibitors of class A and C beta-lactamase enzymes derived from Escherichia coli (TEM-1) and E. cloacae (P99). As 6-(1-hydroxyethyl) derivatives 4c and 6c were found to be weak beta-lactamase inhibitors, they were further investigated in combination with amoxicillin against a series of beta-lactamase-producing bacterial strains. Some structure-activity relationships are discussed.

6-(1-Hydroxyalkyl))penam sulfone derivatives as inhibitors of class A and class C beta-lactamases II

Two stereoselective processes for the synthesis of novel 3,6-disubstituted penam sulfone derivatives were developed. One 6beta-(1-hydroxyethyl) and four 6beta-hydroxymethyl penam sulfone derivatives were synthesized. All four 6beta-(hydroxymethyl)penam sulfone derivatives demonstrated good IC50 against both TEM-1 and AmpC beta-lactamases. Of these, 6beta-hydroxymethyl penam sulfone derivative 25 was the most active inhibitor which was able to restore the activity of piperacillin in vitro and in vivo against both TEM-1 and AmpC beta-lactamases producing organisms.